The present invention relates to a method for purifying phosphate containing waste waters in which aluminum oxide is used as the sorption material.
For the removal of phosphates from waste waters, for example, from waste discharges from biological clarification systems, the prior art practice has been flocculation with iron or aluminum salts. A drawback of the flocculation process is that there is a great consumption of salts and possibly of means or agents which aid flocculation. Moreover, in the flocculation process, a large quantity of mud or sludge is produced which must be disposed of. Possible recovery of the phosphates from these muds has not as yet been effected and is very costly because of the relatively low concentration in the mud.
There are literature references which indicate that phosphate ions are sorbed by aluminum oxide. Two process principles have become known in this connection.
In the first process, the percolation of the waste water is effected through a column filled with coarse Al.sub.2 O.sub.3 grains, for example, grain sizes of from 1 to 5 mm. In the second process, a fluidized bed is used with fine-grained Al.sub.2 O.sub.3 having grain sizes of, for example, 0.05 to 0.2 mm.
These methods which employ aluminum oxide so far have not as yet been used, other than in laboratories, and have a number of drawbacks. With respect to the first process principle where use is made of a fixed bed, the columns are clogged very quickly due to the development of mud. The columns must therefore be rinsed out very often. Moreover, abrasion of the coarse grained Al.sub.2 O.sub.3 produces relatively heavy losses. With respect to the second process principle where use is made of a fluidized bed, the mode of operation of fluidized bed reactors is very irregular. Further, it happens that unpurified or incompletely purified water breaks through and so-called bubble formation takes place. In addition, the adsorption kinetics in the fluidized bed are very poor due to great film thicknesses of the materials to be removed at the individual grains of the sorption agent. Moreover, the start of operation after shut-down in a fluidized bed is very difficult since the fine-grained Al.sub.2 O.sub.3 cakes together when settling and renewed fine distribution is very difficult.